Thick film dielectric electroluminescent displays have been developed and are described, for example, in Applicant's U.S. Pat. No. 5,432,015 (the entirety of which is incorporated herein by reference). These thick film dielectric electroluminescent displays provide for superior resistance to dielectric breakdown as well as a reduced operating voltage compared to thin film electroluminescent (TFEL) displays. A thick film dielectric structure deposited on a ceramic substrate withstands higher processing temperatures and facilitates annealing of phosphor films at higher temperatures to improve their luminosity. With these advantages and, with recent advances in blue-emitting phosphor materials, such displays have achieved the luminosity and color coordinates required to achieve the technical performance of traditional cathode ray tube (CRT) displays. Nevertheless, further improvement is desirable to simplify adjustment of the display color balance, to facilitate maintenance of the proper color balance of the display over its operating life and to simplify the process used to manufacture the displays to reduce cost.
Cerium-activated strontium sulphide phosphor materials are traditionally used in electroluminescent displays for blue color, while manganese-activated zinc sulphide materials are employed for red and green colors. The optical emission from these phosphor materials must be passed through an appropriate chromatic filter to achieve the necessary color coordinates for red, green and blue sub-pixels, resulting in a loss of luminance and energy efficiency. The manganese-activated zinc sulphide phosphor has a relatively high electrical to optical energy conversion efficiency of up to about 10 lumens per Watt of input power while a cerium-activated strontium sulphide phosphor has an energy conversion efficiency of 1 lumen per Watt, which is relatively high for blue emission. Optical filters must be used with these phosphors because the spectral emission for these phosphors is quite wide, with spectral emission for the zinc sulphide-based phosphor material spanning the color spectrum from green to red and that for the strontium sulphide-based material spanning the range from blue to green. The spectral emission of the cerium-activated strontium sulphide phosphor can be shifted to some degree towards the blue by controlling the deposition conditions and activator concentration, but not to the extent required to eliminate the need for an optical filter.
Blue light emitting phosphor materials have been developed having narrower emission spectra. These phosphor materials include europium-activated barium thioaluminate compounds which provide good blue color coordinates. The stability of europium activated barium thioaluminate phosphor materials has been further improved by judicious addition of oxygen to the phosphor during phosphor film processing, as disclosed in Applicant's co-pending International Patent Application PCT/CA03/00568 filed Apr. 17, 2003 (the disclosure of which is incorporated herein in its entirety). This improvement facilitates blue phosphor life which commensurates with commercial requirements, but still allows for a decrease in the blue light luminance to 50% of its initial value over the operating life. This decrease, relative to the decrease in the luminance of red and green electroluminescent phosphors, must be taken into account in maintaining the desired color balance of an electroluminescent display over its operating life. In general, the luminance of the red, green and blue electroluminescent phosphors that make up the sub-pixels of the display decrease at different rates and result in a shift in the color balance of the display as it ages. This shift can be compensated, to some extent, by the driving circuitry. For instance, if the rates of luminance decay of the different colors are predictable, or if sensors are incorporated to measure the sub-pixel luminance at different points in the display life, adjustments to the driving voltages to the sub-pixels can be made. These measurements, however, add complexity and cost to the manufacture and operation of the display.
In addition, each red, green and blue electroluminescent phosphor in a display will each have a specific threshold voltage whereby each begins to luminesce. These specific threshold voltages must each be carefully matched to each individual phosphor to minimize display power consumption. If these voltages are not matched properly, the brightness ratios between red, green and blue will be incorrect. Such matching requires precise control over the thickness and composition of the phosphor and adjacent dielectric layers within the display to the extent that manufacturing yield may be compromised.
The process of forming a patterned phosphor structure used to define individual sub-pixels for a color electroluminescent display is described in Applicant's International Patent Application WO 00/70917 (the disclosure of which is incorporated herein in its entirety). The patterning process requires photolithographic processes involving photoresist deposition, exposure, phosphor film etching and phosphor film lift-off processes for each sub-pixel phosphor material, which involves many successive steps and leads to relatively high manufacturing costs. The chemicals used in such photolithographic processes have to be carefully purified and their use carefully controlled to avoid damage to the typically moisture-sensitive phosphor materials during the patterning process, which can also add to the cost of display manufacture.
Colour organic light emitting diode (OLED) displays are known and described, for example, in the following references: T. Shimoda et al., Society for Information Display 99 Digest, pp 376-80; U.S. Patent Application 2002/0043926; C. Hosokawa et al., Society for Information Display 97 Digest pp 1073-6, and U.S. Pat. No. 6,608,439. The OLED described in U.S. Pat. No. 6,608,439 uses semiconductor nanocrystal layer or layers to produce different colours. OLEDs, however, cannot be used to build a passive matrix, large area display having many rows of pixels with a reasonable luminance. This limitation may be mitigated to some extent by using active matrix addressing, but the thin film transistor (TFT) array needed for active matrix addressing is, in itself, difficult to scale up and costly for large area displays with a large number of addressable rows.
U.S. Pat. No. 5,670,839 describes an electroluminescent device that utilizes photoluminescent materials to convert ultraviolet light to visible light. The conversion efficiency for converting ultraviolet light is relatively low. In addition, ultraviolet light tends to degrade the display.
It is therefore highly desirable to provide a color electroluminescent display in a cost effective and operationally effective manner that obviates the shortcomings of the prior art.